Your fish isn't necessarily going to DIE without a heater, but it'll go step by step towards that direction... kinda. It's more like: He gets lethargic, and is more susceptible to disease. His immune system weakens. Etc, etc.

Alienbetta ... how in the world would he ever make a nest down there. Make sure it's not simple air bubbles.

He grabbed the rocks and put the around like in a ring and he would sleep in it!

this heater only raises the water a few degrees, and I used it for a week in a 2-3 gallon tank-not great-that's why i ordered my stealth-it is great-has been set on 82 degrees and works like a charm.
I don't need the 7.5 hydor anymore so if anyone needs it pm me.

I will be getting a betta soon and have a 2.65 tank with a crappy filter that I might change. Is there any mini heater that won't boil my fish(heat to a certain temparature then cool off and when it gets cold again it heats back up)? Also, can you explain the whole cleaning the tap water with so many things to me? I really appreciate it. Thanks

Just saying, the tetra heater i got from Walmart didn't work and i got the same looking heater form petco I think it was by hydor and it did not work either.
I would recommend the Marineland Visi Therm Stealth 25 watt for a good heater for a 3 gallon.
As for your the questions, I would post them on a new thread so people can help you
good luck!